Spectrometers are used in many applications for measuring properties of light across a range of wavelengths. For example, a spectrometer can be used for compositional analysis, by obtaining absorption or emission spectra for an object of interest. The presence and location of peaks within the spectra can indicate the presence of particular elements or compounds. Spectrometers are commonly used for analysis at optical wavelengths, but can also be used at other wavelengths such as microwave and radio wavelengths.
Spectrometers are typically relatively complex and expensive devices that require the alignment of a number of moving parts to be controlled with high precision. For example, a typical spectrometer may focus light onto a diffraction grating to split an incident beam into separate wavelengths, and the diffraction grating may be rotated to a specific angle to direct light of a particular wavelength towards a detector. In recent years chip-based spectrometers have been developed which can be highly miniaturised, have no moving parts, and can be manufactured using well-established lithography techniques.
A typical chip spectrometer, which may also be referred to as a spectrometer-on-a-chip, comprises a substrate onto which are patterned a waveguide and a plurality of disk resonators coupled to the waveguide. The waveguide guides the input light to the disk resonators. Light is input to one end of the waveguide, and each resonator is arranged to support a resonant mode at a particular wavelength such that only light of that wavelength is coupled into the resonator. On top of each disk resonator is an electrode for detecting current that is proportional to the amount of light present in that resonator. The current detected in each resonator therefore indicates the amount of light at that wavelength that was present in the input beam of light. Each electrode is further connected to a signal bond pad for connecting the spectrometer to an external device for measuring the current. To ensure that light input to the waveguide is absorbed by the disk resonators and not by the waveguide, the disk resonators and waveguide have to be constructed to have different properties, for example by ensuring that the semiconductor band gap in the waveguide is higher than the band gap in the disk resonators. The need for different band gaps adds to manufacturing complexity due to the fact that additional epitaxial re-growth and processing steps are required.